Various techniques for detecting respiratory sleep disorders are known. These techniques implement a variety of sensors. For example EP-A-0 970 713 and its counterpart, U.S. Pat. No. 6,574,507, commonly assigned herewith to ELA Médical, describe diagnosis of the occurrence of an apnea starting from the minute-ventilation signal (the “VE signal,” sometimes referred to as the “MV signal”), which is a parameter with physiological preponderance generally obtained by measuring a trans-thoracic impedance, giving a continuous indication of the respiration rate of the patient.
The detection of the apnea or of hypopnea makes it possible to diagnose conditions such as the syndrome of sleep apnea (SSA), obstructive apnea, or central apnea, which is a pathology likely to involve a number of disorders, such as diurnal hyper somnolence, heart rate disturbances, or hypertension. SSA can be defined by a significant recurrence of apnea or hypopnea associated clinical signs. An “apnea” or “respiratory pause” is a temporary pause of the respiratory function lasting longer than 10 seconds and occurring while the patient is sleeping. An “hypopnea” is defined as a significant decrease of the minute-ventilation, for example, a decrease of more than 50%, compared to an average of former reference, but without pause of the respiratory function (the minute-ventilation is the product of the amplitude multiplied by the frequency of the successive respiratory cycles).
The measurement of the minute-ventilation necessary for this detection of the apnea or of hypopnea is carried out by injecting of impulses of a constant current of a few hundred microamperes at a frequency of some Hertz between two electrodes laid out in the rib cage of the patient, or between the case of the implanted device and an electrode, for example, a stimulation electrode. The variations of impedance reproduce the variations of thoracic volume, with peaks of impedance at the time of an inspiration, i.e., when the lungs are filled with air, and a decreasing impedance over time as the air is expired from the lungs.
It was noted in clinical studies that a system implementing a respiratory activity sensor recording variations of pulmonary volume at the thoracic level can be deluded in certain circumstances, leading to risks of false positive and false negative readings that can disturb interpretation of the signals by the device and thus lead to an erroneous diagnosis. Some of these disturbances can be eliminated by known adapted filtering techniques.
The trans-thoracic impedance varies according to the resistivity of tissue at the time current impulses are injected. As this resistivity depends primarily on the quantity of air in the lungs and the quantity of blood in the cardiac cavities, the impedance signal collected is modulated at the same time by breathing and the heart rate of the patient. The impedance is also modulated by variations in spacing between the measurement electrode and the case of the implanted device, which can change as the heart beats. These cardiac components can be eliminated by simple low-pass filtering, the heart rate being in the majority of the cases about three times higher than the respiratory frequency. In addition, in order to extract from the signal only the respiratory component (only the dynamic variation is significant), it is advisable to eliminate any static impedance related to the impedance from tissue recorded in a stable body position and in the absence of any breathing and cardiac beat. This static component can be eliminated by implementation of a high-pass filter.
When the trans-thoracic impedance is used to estimate the minute-ventilation and to control the heart rate, this estimate of the minute-ventilation is based on an average of several successive respiratory cycles, so that the impact of some respiratory cycles with artifacts remains low.
The starting point of the present invention lies in the observation that certain phenomena, which are not eliminated by existing low-pass and high-pass filtering, are able to disturb measurement of the trans-thoracic impedance. In particular, if one wishes to use the trans-thoracic impedance to follow instantaneous variations in respiratory activity, in particular to detect apnea and hypopnea, it is essential to eliminate all the cycles with artifacts, at the risk of producing false positives and false negatives, that can lead to erroneous diagnoses of respiratory disorders of the patient. Thus, the trans-thoracic impedance can be modified by a movement of the patient, or varied by contractions of the diaphragm during an obstructive apnea. These phenomena can produce artifacts that will disturb the system and can lead to erroneous detection of particularly full or fast respiratory cycles, or of low amplitude or long periods, which can lead to a false positive.
Another type of artifact can result from the presence in the impedance signal of a component related to cardiac beats that is not eliminated by existing high-pass filters. Indeed, in certain circumstances (for example, if bradycardia and hyperventilation occur at the same time), the respiratory frequency and the heart rate can become sufficiently close that the cardiac beats influence the impedance signal to a significant degree. The heart rate can then be incorrectly interpreted as the respiration rate, which may mask the presence of an hypopnea or an apnea (a false negative, at the time a true respiratory pathological event occurs).
When external equipment is being used (e.g., polysomnography), these disturbances can be detected by the person interpreting the signals, who can decide not to take into account one or more channels when the signal appears disturbed. This subjective appreciation of the signals is not possible in an implanted apparatus, however, which functions in an entirely automatic way according to rules defined by an analysis algorithm.